I have read whole mushrooms should be washed before cooking. I neither wash them nor brush them. What is the main reason behind doing these?
"Should You Wash Mushrooms You’re Going to Cook? If they’re whole, yes. Although many sources advise against washing mushrooms (to avoid their soaking up any additional moisture) and suggest brushing them instead, once we learned that mushrooms are over 80 percent water we began to question their ability to absorb more liquid. To find out, we rinsed a batch in cold water, weighing them before and after their wash. We found that six ounces of mushrooms gained only about a quarter ounce of water, and most of this was beaded on the surface.
Cut mushrooms are a different story. The exposed flesh will absorb water like a sponge, so rinse mushrooms before slicing them. And be careful not to wash mushrooms until you are ready to cook them or they will turn slimy."
Mushrooms 101: Everything You Need to Know - Cook's Illustrated
"Should You Wash Mushrooms You’re Going to Cook? If they’re whole, yes. Although many sources advise against washing mushrooms (to avoid their soaking up any additional moisture) and suggest brushing them instead, once we learned that mushrooms are over 80 percent water we began to question their ability to absorb more liquid. To find out, we rinsed a batch in cold water, weighing them before and after their wash. We found that six ounces of mushrooms gained only about a quarter ounce of water, and most of this was beaded on the surface.
Cut mushrooms are a different story. The exposed flesh will absorb water like a sponge, so rinse mushrooms before slicing them. And be careful not to wash mushrooms until you are ready to cook them or they will turn slimy."
Mushrooms 101: Everything You Need to Know - Cook's Illustrated